1. Field of the Invention
The present invention is directed to the field of acoustic transducers including magnetic acoustic transducers and, more particularly, to the treatment of conductor circuits associated with the diaphragms of magnetic acoustic transducers. The transducer diaphragm is mounted in proximity to magnets fixed to support frames such that when current flows in the conductor circuit the resulting electromotive forces vibrate the diaphragm with piston-like motion and accurately reproduce acoustic signals.
2. History of the Related Art
Magnetic acoustic transducers and particularly planar magnetic loudspeakers are generally popular because of their good sound reproduction characteristics. Such loudspeakers typically include a generally flat diaphragm having a pattern of one or more conductors attached which form the "voice coil" or signal current carrying conductors. The diaphragm is positioned so that the conductors are attracted and repelled by the magnets as current signals pass through the conductors, thereby causing the diaphragm to oscillate and produce sound.
The sound reproduction of a typical planar magnetic transducer is sensitive to the operating characteristics of the diaphragm. A typical planar diaphragm includes a thin flat polymer membrane with a pattern of thin foil-like conductors attached to the membrane. Aluminum is an optimum conductor material because of its light weight and low electrical impedance. To obtain optimum acoustic response, the diaphragm is held under in-plane tension generally in a plane parallel to the pole faces of one or more magnets. The path for the electrical conductor runs on the diaphragm is generally chosen so the current flowing through the conductors induces net forces of uniform direction for all of the conductor segments or runs within what is referenced as an "active area" of the diaphragm. The generated forces thereby cause the general direction of diaphragm motion to be perpendicular to the diaphragm surface during operation of the transducer. The "active area" of the diaphragm, as described and referenced throughout this application, both in the specifications and claims, is that area of the diaphragm which is not constrained from motion by the rigid frame which supports the diaphragm relative to the one or more magnets. The mechanical properties of the diaphragm and conductor pattern determines modal behavior patterns of the diaphragm, and hence its sound reproduction characteristics.
Heat is produced within the conductor pattern and can modify the acoustic output of the transducer when power is applied to the electrical conductor circuits from a conventional amplifier. The problem is more severe during operation of smaller planar magnetic transducers as the area of heat dissipation of the conductors is reduced. The sound reproduction characteristics of smaller planar magnetic speakers are also more sensitive to diaphragm tension. At increased electrical power, the aluminum conductor material expands at a much greater rate than the polymer diaphragm, which is typically made of MYLAR.TM.. The resulting shear stress from differential thermal expansion between the two layers may result in non-uniform stresses across the diaphragm. The diaphragm tension may thus change over parts of the "active area" of the diaphragm and undesirable audible distortion may occur. As the size of a planar magnetic transducer is reduced, the heat buildup is greater for a given amount of input power because of the reduced surface area of the diaphragm. The heat buildup reduces the maximum usable power which may be delivered to the transducer. Increased audible distortion and reduced efficiency resulting from changes in the diaphragm tension have been observed in the past with smaller planar magnetic transducers. The changes in tension due to heating from increased electrical power through the conductor pattern are dependent on the materials, layout, and cross-section of the diaphragm structure. The non-uniform displacement also causes a non-piston like behavior of the diaphragm creating valleys and peaks in the frequency response of the loudspeaker. For example, local pockets may form in the diaphragm which move out of phase with respect to the pistonic motion of the rest of the diaphragm, resulting in distortion and loss of output.
Ideally, the behavior of the diaphragm is like a piston through the normal operating frequency range.
Diaphragm structures common in the art include circuits etched into adhesively-backed aluminum laminated to a polymer diaphragm, and circuits with conductive wire or strips adhesively bonded to a polymer diaphragm. For these diaphragms with an adhesive laminated between the conductor and polymer diaphragm, as the heating increases further, random sections of the conductor may start to delaminate in response to the thermal stress induced in the adhesive layer. The random delamination of the conductor produces sites of thermal stress relief. The location and size of these non-adhered sites are not controlled, resulting in variable acoustical response of the diaphragm. Also, damage to the underlying polymer film is possible, and large non-adhered areas typically form an arch which can limit full excursion of the diaphragm. Hence, uncontrolled separation of the conductor from the diaphragm due to heating stresses creates thermal stress relief sites of varying size and allows higher operating powers, but may reduce optimum sound reproduction quality due to increased distortion and limited diaphragm excursion. Speakers with diaphragms which delaminate randomly are subject to large variations in operating characteristics and reduced yield of operational speakers.
The invention reduces the local shear stress due to differential thermal expansion and allows for higher conductor temperatures and hence increased power handling. Prior art inventions increased the maximum operating power in electroacoustic speakers by alternative methods including improved heat dissipation from the conductors on the diaphragm. These magnetic acoustic transducers using the prior art alternative designs have limitations in sound reproduction characteristics. U.S. Pat. No. 4,281,223 to Ugaji et al, discloses two heat-resistant vibratory structures for a ribbon type tweeter. In the first, a heat-resistant film containing polyimide is coated on the conductor-diaphragm surface and by improved chemical adhesion to the conductor increases heat conduction from the conductors. However, the diaphragm thickness is approximately doubled compared to typical Mylar.TM.-aluminum diaphragms. Due to the added mass, this method is suited to a miniature tweeter-type planar magnetic transducer, but in larger mid-range planar magnetic transducers there is reduced efficiency due to the increased mass of the diaphragm. Another structure shows use of the heat-resistant coating itself as the substrate portion of the diaphragm; however, the complicated procedure and long processing time is not suited to mass production. The U.S. Pat. No. 4,264,789 to Kaizu et al, discloses a metallic plate or layer at the circumferential portions of the diaphragm and spaced within 100 um for the purpose of heat conduction from the voice coil. This extra layer adds mass and decreases efficiency of the transducer. For diaphragm circuits using vapor-deposited aluminum voice coils, it is difficult to produce suitable conductor thicknesses for optimum electrical characteristics and these circuits are limited in power-handling capability.
Hence, the known methods of increasing the maximum electrical power rating of a planar magnetic transducer have the limitations of reduced efficiency, and slow, costly processing.
While the diaphragm of the magnetic transducer examples described previously are substantially planar, this need not be the case as the diaphragm could also be curved. Also, the invention is not limited to use with small planar magnetic transducers as the invention could be useful in larger planar magnetic transducers.